1. Field of the Invention
This invention relates to a washing machine provided with a direct drive type electric motor for directly driving a rotatable tub and an agitator.
2. Description of the Prior Art
Conventional full automatic washing machines comprise a rotatable tub rotatably mounted in an outer tub and serving both as a wash tub and as a dehydration or drying basket and an agitator mounted in the rotatable tub. A single electric motor is provided for driving both of the rotatable tub and the agitator. More specifically, in a washing operation, a motor speed is decelerated and its rotation is transmitted only to the agitator so that the same is driven repeatedly alternately in the forward and reverse directions. In a drying operation, the motor speed is not decelerated and its rotation is transmitted both to the rotatable tub and to the agitator so that both of them are rotated at high speeds.
A rotation transmission path from the motor to the rotatable tub and the agitator includes a belt transmission mechanism and a gear reduction mechanism having planetary gears in the above-described washing machine. This belt transmission mechanism and gear reduction mechanism increase the weight and the height of the washing machine, resulting in an increase in the size thereof. Furthermore, a loud noise is produced during operation of the gear reduction mechanism.
To solve the above-described problems, the prior art has proposed a direct drive mechanism including a direct drive type motor for driving the rotatable tub and the agitator. The direct drive mechanism includes a clutch for changing between an arrangement where the agitator is directly driven repeatedly in the forward and reverse directions with the rotatable tub being stopped and an arrangement where both of the rotatable tub and the agitator are directly driven at high speeds. Thus, since the rotatable tub and the agitator are driven directly by the motor, the belt transmission mechanism and gear reduction mechanism are eliminated. Consequently, the weight and height of the washing machine can be reduced, and the noise due to the operation of the gear reduction mechanism is eliminated.
The above-described washing machine requires an electric motor which produces a high torque and whose rotational speed is variable in a wide range from a low speed to a high speed. In view of these needs, a brushless dc motor has been proposed to be used in the above-described washing machine. The brushless dc motor is controlled by an inverter and provided with three Hall ICs, for example, for detecting a rotational position of a rotor of the motor. Based on position signals produced by the Hall ICs, the inverter generates a rectangular wave voltage energizing the motor for a period corresponding to an electrical angle of 120 degrees in each one ac cycle. The inverter supplies the generated rectangular wave voltage to a stator winding of the brushless motor to thereby drive the latter. The above-described arrangement, to obtain the rotor position signals from the Hall ICs, is very simple and cost-effective.
Switching a phase to be energized or commutation results in the variations in the torque when the brushless motor is energized from the rectangular wave voltage as in the above-described motor control manner. The torque variations cause vibrations in the brushless motor, resulting in noise due to operation of the motor. The operation noise is reduced to a certain level in the above-described washing machine since it employs the direct drive mechanism. Under this condition, the noise due to the torque variations is conspicuous. Furthermore, the brushless motor is mounted on an outer tub of the washing machine. Accordingly, the outer tub resonates to the vibration of the brushless motor such that the operation noise is increased.